The present invention relates to spectroscopes or spectrometers for detecting a driving position of a main wavelength dispersive device, and particularly relates to spectroscopes having positional detecting structures of the wavelengths.
In known main wavelength dispersive devices for spectrum analysis of light, such as described in GB Pat. No. 2,208,430, a reflection type diffraction grating (main grating) having 1800 to 4800 grooves/mm is used. A plane type diffraction grating (sub-grating) having 10 to 100 grooves/mm is fixed to, and driven together with, the main grating for a sub-wavelength dispersive device for position-detecting the main grating. The positional detection of the main grating means detecting an incident angle of light to be analyzed and/or an emitting angle of spectrum-analyzed light, so that a spectrum-analyzed wavelength detected by a detector can accurately be specified.
An incident slit is incident with the light to be analyzed. The light incident from the incident slit is spectrum-analyzed by the main grating and thereafter emits from an outgoing slit to be detected at a main detector. Then, by suitably driving a driving unit, light with an optionally selected wavelength is allowed to emit to the outgoing slit, enabling the intensity of the light having the optionally selected wavelength to be measured.
On the other hand, light incident on a sub-grating has a narrow wavelength band, such as a laser beam for positional detection. The incident light on the sub-grating is diffracted by the sub-grating, and thereafter is incident on the sub-detector having a plurality of detecting elements aligned in a row in the analyzing direction. With the smaller number of grooves of the sub-grating, the diffraction light having a plurality of diffraction orders is diffracted by the sub-grating.
An interval of the diffraction light incident on the sub-detector is determined in accordance with a driving position, the number of grooves of the sub-grating, wavelength of a light source, and a distance from the sub-grating to the sub-detector, and the like.
For this reason, the diffraction light transverses one by one on the sub-detector depending on driving of the sub-grating, in addition, each interval of the diffraction light and the width of the sub-detector are set so that one diffraction light is always incident on the sub-detector, and the main grating and the sub-grating are fixed and simultaneously driven to enable positional detecting of the wavelength dispersive device (main grating) by using the diffraction light.
In case of fixing the main grating with the sub-grating, affixing on the same plane simplifies designing, manufacturing, driving operation, and positional detecting control, this therefore is a reason of fixing it on the same plane.
To raise the accuracy of positional detecting, the size of the detecting element of the sub-detector may preferably approximate a half width of intensity distribution of the diffraction light. The smaller number of detecting elements is more preferable in view of the data processing. The width of the sub-detector is determined depending on a size of the detecting element and the number of such elements.
On the other hand, the width of the sub-detector is always required to be incident one or more diffraction lights. Thus, the interval and the half width of intensity distribution of the diffraction light may preferably be constant as much as possible. From among these, the half width is substantially constant.
Generally in the diffraction grating, the larger the diffraction angle, the larger the angle distribution of diffraction light, and the larger the interval of the diffraction light provided. In particular, as one approaches an upper limit of the diffraction range, it becomes substantially larger, and approaches as much as ten times that associated with a smaller diffraction angle.
When fixing the main grating with the sub-grating on the same plane, the drawback is that, in case of spectrum-analyzing light having a wavelength capable of enlarging the diffraction angle of the main grating, the diffraction angle of the sub-grating comes larger and the interval of the diffraction light is also enlarged.